1. Introduction

Molecule-based magnetic materials,such as single-molecular magnets (SMMs),photomagnets,magneto-optical materials, magnetic conductors,ferroelectromagnets,chiral magnets,and multifunctional materials have attracted increasing interest over the last few years ^[1]. The design and synthesis of SMMs is of particular interest,for their magnetic bistability potentially allows devices for ultimate high-density memory storage and quantum computing ^{[2, 3]}. Manganese based SMMs have been studied the most,with focus on the paramagnetic nature of the manganese ion in various oxidation states,which provides interesting magnetic properties ^{[4, 5, 6]}. Detailed studies of the solid state and frozen solution magnetic properties of a family of oxime-based hexanuclear Mn(III) SMMs (Mn6 SMMs) led to the first established solid state magnetostructural correlations for any SMM,demonstrating that their solid state magnetic properties are crucially dependent on very small changes in geometry; in particular,the Mn-N-O-Mn torsion angles of the metal oxime core ^{[7, 8]}.

We are interested in introducing other transition metal ions into the Mn6 system and studying how structure change affecting the magnetic properties. In this paper,we report the synthesis, structural characterization,and magnetic properties study of a bimetallic hexanuclear cluster based on 5-chlorosalicylaldehyde oxime (Cl-H₂Sao),[Mn₄Ni₂O₂(Cl-Sao)₆·(CH₃OH)₈]·10CH₃OH.

2. Experimental

All chemicals and solvents were commercial products and used without further purification. Elemental analyses of C,H,and N were performed by a Carlo Erba 1106 elemental analyzer. Metal analyses of Mn and Ni were performed by ICP-AES. IR spectra were recorded as KBr pellets on a Nicolet 750 FTIR spectrophotometer. Magnetic data were recorded using a Quantum Design SQUID magnetometer. To avoid orientation in the magnetic field,the samples were pressed in a home-made Teflon sample holder equipped with a piston. The data were corrected for diamagnetism of the constituent atoms using Pascal’s constants. The X-ray diffraction measurements for 1 were collected on a Bruker Apex SMART CCD system equipped with Mo Ka radiation (λ= 0.71073 ) at room temperature. Cell constants and an orientation matrix for data collection were obtained by leastsquares refinement of the diffraction data from 7497 unique reflections. The structure was solved by direct methods and refined by a full matrix least squares technique based on F² using the SHELX program ^[9]. The crystallographic data for the four compounds are summarized in Table 1,and selected bond lengths and torsion angles are list in Table 2.

Table 1

Table 1 Crystal data and structure refinement of 1.

Table 1 Crystal data and structure refinement of 1.

Table 2

Table 2 Selected bond lengths ( ) and torsion angles (°) for 1. Symmetry transformations used to generate equivalent atoms: #1 -x+2,-y,-z.

Table 2 Selected bond lengths ( ) and torsion angles (°) for 1.

To pale pink solutions of Mn(ClO₄)₂·6H₂O (1 mmol) in MeOH (20 mL) were added equivalent amounts of the Cl-H₂Sao (1 mmol, 10 mL),and five times of Et₃N (0.7 mL). After 30 min of stirring,the Ni(ClO₄)₂·6H₂O (0.5 mmol) in MeOH (20 mL) was added. The solutions were left stirring for 30 min,filtered,and then left to slowly evaporate. Dark green single crystals suitable for X-ray diffraction study were obtained after 2 days. Elemental anal. calcd. (found) for 1 (formula C₃₀H₄₅C_l3Mn₂N₃NiO₁₆): C 36.82 (36.75),H 4.64 (4.44),N 4.29 (4.34) %. ICP result calcd. (found): Mn,11.0 (11.2); Ni 6.2 (6.0) %. IR (KBr pellet) = 416.1 (m),466.6 (m),533.1 (w),626.8 (s),648.1 (s),679.5 (vs),749.5 (s),789.4 (w),827.8 (w), 856.8 (w),918.5 (vs),1026.1 (vs),1098.6 (m),1124.4 (m),1153.9 (m),1170.8 (m),1202.5 (s),1277.0 (vs),1327.8 (w),1392.5 (vs), 1440.0 (s),1471.4 (m),1527.9 (s),1597.5 (vs),3392.3 (s) cm^-1.

3. Results and discussion

Single crystal X-ray diffraction analysis reveals that the complex 1 (Fig. 1),like most Mn6-Sao systems,consists of two off-set [Mn₂NiO]⁶⁺ triangles lined together via two oximate oxygen atoms from two η1:η2:η1:μ3 Cl-Sao^2- ligands and two phenolate oxygen atoms derived from two h2:h1:h1:m3 Cl-Sao^2- ligands. The remaining two oximato(-2) ligands each bridge one Ni^II ion in an η1:η1:η1:μ fashion,thus forming a [Mn^III ₄Ni₂(μ₃-O)₂(μ₃- ONR)₂(μ-ONR)₄(μ-OR0)₂]⁴⁺ core. The remaining axial coordination sites on the Mn and Ni ions are filled by methanol molecules. All MnIII and NiII ions display a six-coordinate distorted octahedral geometry,and the Jahn-Teller axes of all three ions are nearly parallel to each other and are roughly perpendicular to the [Mn₂NiO]⁶⁺ plane. The oxidation states for the two manganese(III) ions and nickel(II) ions were established by charge-valence considerations,bond length distances,and bond valence sum (BVS) ^[11] calculations with the obtained values of 3.24,3.22 and 2.16 for Mn1,Mn2 and Ni1,respectively. The nearest distance between Mn ions different from Mn4Ni2 cores is 9.659 ,Mn-Ni distance is 9.087 ,and Ni-Ni distance is above 10 ,which indicates that the inter-cluster magnetic interaction can be neglected. Adjacent molecules are linked by intermolecular H bonds to form a 1D infinite chain structure (Fig. 2),and there is no obvious π-π stacking interaction between phenolate.

Fig. 1

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Fig. 1.Perspective view of the complex 1 (30% thermal probability ellipsoids,for clarity hydrogen atoms and uncoordinated methanol molecules are omitted).

Fig. 2

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Fig. 2.Molecular packing view of 1 along the b direction; the intermolecular hydrogen bonds and weak bonds are shown as dashed lines.

Solid-state DC (direct current) magnetic susceptibility (χ_M) data were collected in a 0.1 T field in the 1.8-300 K range (Fig. 3). The χ_MT value for 1 decreases steadily from 11.59 emu K mol^-1 at 300 K to 5.47 emu K mol^-1 at 35 K,and then decreases sharply to 1.14 at 1.8 K. The χ_MT value at 300 K for 1 is lower than 14 emu K mol^-1,the value expected for a Mn4Ni2 complex with non-interacting metal centers between Mn and Ni ions. The decrease of the χ_MT value with decreasing temperature also indicates antiferromagnetic coupling within complex 1. The rapid decrease observed in the χ_MT value at 35-1.8 K range is most likely due to zero-field splitting effects and perhaps weak intermolecular interactions mediated by the hydrogen bonding in the crystal structure. The non-superposition of the M-H data (Fig. 4) on a single master curve suggests the presence of magnetic anisotropy and antiferromagnetic interaction on 1.

Fig. 3

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Fig. 3.Plot of χMT vs. T for 1. The solid line represents the best fit of the experimental data. Insert: the interaction between Mn and Ni ions (red ball,oxygen; blue ball,nickel; green,manganese).

Fig. 4

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Fig. 4.Plot of M Y. Zhang et al. / Chinese Chemical Letters 25 (2014) 937-940 939 vs. H for 1.

The magnetic susceptibilities are fitted by least-squares fitting procedure with Levenberg-Marquardt algorithm using the program MAGPACK ^[10].

AC magnetic susceptibility measurements were collected on complex 1 with an oscillating magnetic field of 3.5 Oe in the frequency range 10-1000 Hz and at temperatures between 1.8 and 6 K to probe for any slow magnetic relaxation associated withSMM behavior. No out-of-phase susceptibility ( χ") was observed for the complex 1 above 2 K. We speculated that the location of Ni ions may affect the SMM property of the cluster. If the Ni ions occupy the position of other two Mn ions,the cluster may experience magnetic relaxation at temperatures between 1.8 K and 6 K.

4. Conclusion

In summary,the bimetallic hexanuclear cluster based on 5- chlorosalicylaldehyde oxime has similar structure [M₆O₂] core to Mn6-Sao system. Magnetic investigation reveals that 1 exhibits an antiferromagnetism behavior. No SMMs properties were found in complex 1.

Supplementary materials

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center,CCDC 961675,for complex 1. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html.